A thermal base precursor is a neutral or weakly basic compound that can generate a strong base during thermal processing. Various base precursors are known as, for example, described in U.S. Pat. Nos. 3,220,846, 4,060,420 and 4,731,321. Japanese Patent Application No. 1-150575 describes thermally-releasable bis-amines in the form of their bis(aryl sulfonylacetic acid)salts. Other amine-generating compounds include 2-carboxycarboxamide derivatives disclosed in U.S. Pat. No. 4,088,469, hydroxime carbamates disclosed in U.S. Pat. No. 4,511,650 and aldoxime carbamates disclosed in U.S. Pat. No. 4,499,180. Examples of some thermal base precursors are shown in Table III of U.S. Pat. No. 5,258,274 to Helland et al., including cations and anions, which patent is incorporated by reference.
Further examples of base precursors include salts of carboxylic acids and organic bases as described in U.S. Pat. No. 3,493,374 (triazine compounds and carboxylic acids), British Patent 998,949 (trichloroacetic acid salts), U.S. Pat. No. 4,060,420 (sulfonylacetic acid salts), JP-A-59-168441 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) (sulfonylacetic acid salts), JP-A-59-180537 (propionic acid salts), JP-A60-237443 (phenylsulfonylacetic acid salts substituted by a sulfonyl group), and JP-A-61-51139 (sulfonylacetic acid salts).
Base precursors consisting of carboxylic acids and organic di or tetra-acidic bases are disclosed in JP-A-63-316760 and JP-A-1-68746 (corresponding to U.S. Pat. No. 4,981,965). In these base precursors, the activity on heat treatment at 140° C. is compatible with the storability. EP0708086 discloses selected base precursors that simultaneously provide both satisfactory storability and activity on heat treatment at 120° C. or less. These base precursors can be employed when it is desirable to rapidly release a base at a low heating temperatures while maintaining good storability prior to thermal processing. Such bisguanidine salts are selected from the group consisting of a 4-(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′bis(1,3-diethylguanyl)ethylenediamine, a 4(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′-bis(1,3diisopropylguanyl)ethylenediamine, 4(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′-bis-(imidazoline-2yl)ethylenediamine, and other specified compounds.
A base precursor typically has an inherent decomposition point. However, in practical applications, rapid decomposition of the base precursor (the release of the base) is expected only at heating temperatures much higher than its decomposition point. For example, although ease of the decomposition also is dependent on methods of heating, a base precursor having a decomposition point of about 100° C. or less might be subjected to a heating temperature of 120° C. in order to obtain rapid decomposition.
U.S. Pat. No. 4,981,965, hereby incorporated by reference, describes base precursors comprising arylsulfonylacetic acid salts of guanidine bases. Such base precursors have a stable crystal structure, which crystal structure is kept until it melts or is dissolved at an elevated temperature. At the same time that the crystal structure is broken, the carboxylic acid is rapidly decarboxylated to release a base. In these systems, thermolysis of the salt results in decarboxylation to form an arylsulfonylmethyl anion. This anion abstracts a proton from the guanidinium salt to release the free base. This base can then provide the alkalinity required for a number of image-forming processes.
U.S. Pat. No. 4,060,420, hereby incorporated by reference, describes the use of ammonium salts of arylsulfonylacetic acids as activator-stabilizers in photothermographic systems. In these systems the ammonium species is always a protonated basic nitrogen, and thus has at least one labile hydrogen atom. U.S. Pat. No. 4,731,321 discloses ammonium salts of arylsulfonylacetic acid as base precursors in heat-developable light-sensitive materials.
Japanese Patent Application No. 1-150575 discloses thermally releasable bisamines in the form of their bis(arylsulfonylacetic acid) salts. Other amine-releasing compounds include 2-carboxycarboxamide derivatives disclosed in U.S. Pat. No. 4,088,496; hydroxylamine carbamates disclosed in U.S. Pat. No. 4,511,650; and aldoxime carbamates disclosed in U.S. Pat. No. 4,499,180.
As indicated above, it is usually desirable for a thermal base precursor to exhibit good stability during storage but to quickly decompose to form a base when it is heated at the temperature of use. A successful base precursor will not have any adverse effects on the imaging element in which it is contained.